Electrical question....24v to 12v dropper resistor

[ukbaz]

Hi I have a spare 12v jabsco searchlight I have put on the boat aft. Main boat electrics are all 24v and routing back to only 12 volt available is a very awkward job.

Question is what value resistor do I need to put in line to make this work from 24v? I know this is possible as one of the heads on the boat has a 12v toilet running this way from 24 volt supply. The light draws 3.6A.

Any 'bright sparks' out there?

I figured out I'd need a 3.5 ohm 45 watt... but not sure if this is right??

Barry

 

[carlton]

Err...... not sure about this, but couldn't you just locate a 24v. bulb for the torch ?

 

[tinkicker0]

At 12v the light assuming a 50w bulb will draw 4.1watts.

You will need a 24 -12v voltage dropper. This is an heat sink incorporating a zener diode to shed the excess voltage. The heat sink is required to shed the heat created.

You should be able to find a 6A dropper at most truck parts suppliers or your local ICE shop.

 

[Nickcf]

[ QUOTE ]

Question is what value resistor do I need to put in line to make this work from 24v? I know this is possible as one of the heads on the boat has a 12v toilet running this way from 24 volt supply. The light draws 3.6A.

I figured out I'd need a 3.5 ohm 45 watt... but not sure if this is right??

Barry

[/ QUOTE ]

Correct! If the lamp draws 3.6A at 12v then you will need a resistor to drop the other 12v (at 3.6 A) ie about 3.3 ohms and a minimum of 50 watts. However this is a high power resistor and could get very hot so you need to very careful with the location to avoid a fire!
Don't forget though that a 24v system can get to nearly 30v on charge. A stabilised voltage regulator would be a better way to do it ie 24 to 12v stabiliser. Also prevents the brightness varying with the state of the charge (although not really a problem with a searchlight I suppose).

nick

 

[gimmesunshine]

it would be easier and cheaper to wire in another bulb in series

 

[epervier]

I have to go with carlton on this one, it seems you're over engineering a very simple solution, buy a 24volt bulb for the searchlight!!!KISS.

does it look like this one?

http://www.jabscoshop.com/item.asp?ls=24...785&id=3320

 

[markdj]

Please do not use a resistor as this will get hot and is bad bad practice. If you really must then run the light off your 12v socket which should be supplied from your DC-DC converter which you should have for all your 12v instruments. If you don't have one then get one.

The best thing would be to change the bulb to 24v which will half your current consumption - I recommend also to change your head to 24v as you are creating a fire risk and wasting power instead of running off 24v.

 

[nicks]

you would also need to change the motors inside as well to 24v or in the past we have supplied a 24v to 12v converter.

 

[markdj]

True, if it is a motorised job then your only simple option is to plug into your 24-12v converter which all 24v boats should have...